Phabricator has consistently been unable to reach the writable ("master") database while processing recent requests.
This likely indicates a severe misconfiguration or major service interruption.
Phabricator will periodically retry the connection and recover once service is restored. Most causes of persistent service interruption will require administrative intervention in order to restore service.
Although this may be the result of a misconfiguration or operational error, this is also the state you reach if a meteor recently obliterated a datacenter.
This request was served from a replica database. Replica databases may lag behind the master, so very recent activity may not be reflected in the UI. This data will be restored if the master database is restored, but may have been lost if the master database has been reduced to a pile of ash.
In read-only mode you can read existing information, but you will not be able to edit objects or create new objects until this mode is disabled.